Here is a very simplified version of how to digitally paint a portrait using a line drawing as a basis.
I first did a quick line sketch on layout paper using a Design 2 Art Marker. This tool is equipped with two nibs, one fine and a larger one for drawing bold strokes. The sketch was then scanned into the computer.
Using Photoshop Elements 2.0, I added a flat flesh tone with the Paint Bucket Tool.
Now I add tones to the hair and lips.
More inermedate tones are added.
Darker tones are added as well as some modeling to the face.
Sometimes I will flop the image I'm working on. Often portraits can get out of kilter and seeing a flopped view will help detect where you might have gone wrong. I use the Smudge Tool to soften some of the edges.
The finished rough sketch. I added some darker tones to the hair and softened them with the Smudge Tool
You can also easily turn her into a redhead or blond.
Even though this is a rather crude, rough sketch, it's a great way to learn how to render images digitally. Practice ! Practice ! Practice ! Actually, using the Painter program, even the simpler versions, will allow you even more flexibility and rendering options.